Apparatus and process for manufacturing solder balls

ABSTRACT

An apparatus and method of forming fluxless solder balls includes forming solder balls from a supply of solder. A coating is formed on the solder balls for limiting naturally occurring oxide growth on the solder balls before significant natural oxide growth on the solder balls has occurred. The coating allows the solder balls to be soldered without using flux.

BACKGROUND

[0001] The use of small solder balls positioned in ball grid arrays for making electrical interconnections in electronic chip packages is becoming increasingly popular. A typical ball grid array may contain over 1000 solder balls between 12-30 mils in diameter and 50 mils apart. Such a ball grid array allows a large number of electrical connections to be made in a small area. During manufacturing of an electronic chip package which employs solder balls for electrical interconnections, the solder balls are placed in the desired ball grid array configuration upon the chip package at the appropriate location, and then later bonded thereto as well as to any mating surfaces by reflowing the solder balls in a reflow oven. Prior to reflow, flux is applied for chemically removing surface oxides from the solder balls and appropriate surfaces so that the solder balls can be properly bonded thereto. The flux also maintains a protective layer over the cleaned surfaces during soldering and removes reaction products. After soldering, highly corrosive flux residues remain behind which are later removed with solvents in a cleaning process.

SUMMARY

[0002] The present invention provides fluxless solder forms which may be placed in ball grid arrays on chip package substrates. The solder forms are formed from a supply of solder. A layer or coating is formed on the solder forms for limiting naturally occurring oxide growth on the surface of the solder forms. The layer or coating allows the solder forms to be soldered without using flux.

[0003] In preferred embodiments, the solder forms are solder balls or spheres which are formed from molten solder at a solder ball forming station by a droplet spray process. In the droplet spray process, the molten solder is caused to fall as droplets which solidify while falling to form the solder balls. The layer or coating is formed on the solder balls at a coating station while the solder balls are falling and before significant natural oxide growth on the solder balls has occurred. The layer or coating is formed by treating the solder balls with plasma products including fluorine which forms an oxyfluoride layer on the solder balls. The coating station includes a chamber containing the plasma products therein. The solder ball forming station is positioned at the upper end of the chamber. The chamber is supplied with the plasma products from a plasma generator which generates plasma from a gas containing fluorine.

[0004] The fluxless solder forms or solder balls provided by the present invention may eliminate the step of applying flux before soldering and the step of removing flux residues after soldering. This not only reduces manufacturing time but also reduces the inventory of materials and equipment required to be on hand since the flux and the cleaning solvents associated with the eliminated steps as well as corresponding equipment are no longer needed. The elimination of such steps, materials and equipment reduces the manufacturing costs of soldering or reflowing ball grid arrays.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

[0006]FIG. 1 is a schematic drawing of an embodiment of the present invention solder ball apparatus.

[0007]FIG. 2 is a schematic drawing depicting the conversion of an oxide layer to an oxyfluoride layer on a newly formed solder ball when treated with fluorine radicals F⁺.

[0008]FIGS. 3A and 3B are schematic drawings depicting the reaction of fluorine (F) atoms with tin (Sn) atoms to form tin oxyfluorides.

[0009]FIG. 4 is a graph depicting the atomic concentration of oxygen at particular depths for both newly produced solder balls and fluxless solder balls having an oxyfluoride layer.

[0010]FIG. 5 is a graph depicting the atomic concentration of lead (Pb), tin (Sn), fluorine (F), oxygen (O) and carbon (C) relative to depth within 63 Sn/37 Pb fluxless solder balls having an oxyfluoride layer.

[0011]FIG. 6 is a schematic drawing of the solder droplet generator depicted in FIG. 1.

[0012]FIG. 7 is a schematic drawing of the plasma generator depicted in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0013] Referring to FIG. 1, solder ball apparatus 10 is an apparatus which produces treated solder balls 16 that do not require flux when soldering or reflowing. Apparatus 10 includes a solder ball forming station having a solder droplet generator 12 that produces uniform droplets 47 of molten solder which solidify while falling into solder balls 15. The solder droplet generator 12 is positioned within the interior 18 a of a gas tight chamber 18 at the upper portion of the chamber 18. Chamber 18 serves as a coating or treating station in which the solder balls 15 are coated or treated. A plasma generator 14 for generating plasma 13 is coupled to the interior 18 a of chamber 18 by a conduit 28 and supplies chamber 18 with plasma products 13 a for coating or treating the falling solder balls 15. The plasma products 13 a contain atomic fluorine radicals F⁺ that react with the solder balls 15 to form treated or coated solder balls 16 which have an oxyfluoride layer or coating 19 (FIG. 2) thereon. A container 20 is positioned at the bottom of the chamber 18 for collecting the coated or treated solder balls 16. A fitting 24 at the bottom of chamber 18 is connected to a vacuum pump for evacuating the system.

[0014] In use, the interior 18 a of chamber 18 and the interior 30 a of vessel 30 of plasma generator 14 are evacuated to remove gases from the system through fitting 24. After evacuation, nitrogen gas (N₂) is introduced therein and then removed to facilitate the removal of oxygen. The system may be evacuated to a pressure of 0.15 Torr and then filled with N₂ gas to a pressure of 13 kPa before being removed. The introduction and subsequent evacuation of N₂ gas may be repeated to ensure that most of the oxygen in the chamber 18 and plasma generator 14 is removed. Once the chamber 18 and plasma generator 14 are sufficiently evacuated, the plasma generator 14 is turned on and supplied with sulfur hexafluoride gas (SF₆) from a gas source via inlet 26. The SF₆ gas enters plasma generator 14 at a controlled rate for obtaining a desired pressure (for example, 1.5 Torr). The plasma generator 14 dissociates the SF₆ gas to produce a plasma 13 containing highly reactive atomic fluorine radicals F⁺. The following equation describes electron-impact induced molecular dissociation of SF₆ gas:

e+SF₆→SF_(6-x)+XF+e x≦6.   (Eq. 1)

[0015] Preferably, pure SF₆ gas is supplied to plasma generator 14 to obtain the maximum concentration of atomic fluorine radicals F⁺ within the generated plasma 13 for treating solder balls 15. N₂ gas may be mixed with the SF₆ gas to accelerate the decomposition of the SF₆ gas, however, this reduces the concentration of the atomic fluorine F⁺ within the plasma 13. Plasma products 13 a including fluorine radicals F⁺ flow from the inner vessel 30 of plasma generator 14 into the interior 18 a of chamber 18 via conduit 28 which is coupled therebetween.

[0016] The heaters 42 (FIG. 6) of solder droplet generator 12 are turned on to melt solder 38 contained within the crucible 40 of solder droplet generator 12. The solder droplet generator 12 is then operated to form uniform droplets of molten solder 47 which fall downwardly within the interior 18 a of chamber 18 and solidify into solder balls 15 while falling. As the solder balls 15 fall within chamber 18, the solder balls 15 fall or pass through the plasma products 13 a contained therein. The fluorine radicals F⁺ surround and contact the surfaces of each falling solder ball 15. Oxides formed on the surface of the solder balls 15 in a layer 17 (FIG. 2) are treated by the fluorine radicals F⁺. The fluorine radicals F⁺ react with the oxides in layer 17 causing layer 17 to undergo an oxide conversion process which transforms the layer 17 into an oxyfluoride layer or coating 19. In this manner, solder balls 15 become treated or coated solder balls 16. The treated solder balls 16 are collected in container 20 which may contain a quantity of oil 22 such as silicone oil for cooling the solder balls 16. The treatment time of solder balls 15 may be as little as 225 milliseconds. However, the height of chamber 18 may be sized to provide longer treatment times.

[0017] In solder balls 15 having a typical eutectic 63 Sn/37 Pb tin/lead solder composition, the oxide conversion process may be generally described as a conversion from Sn/Pb oxide to Sn/Pb oxyfluoride as follows:

SnPbO_(x)+yF⁺→SnPbO_(x)F_(y).   (Eq. 2)

[0018] More specifically, tin oxides (SnO and SnO₂) are usually the main surface oxide components on untreated 63 Sn/37 Pb solder balls 15. FIGS. 3A and 3B depict the conversion of tin oxides on the surface of solder balls 15 into oxyfluorides by the bonding of fluorine atoms (F) with the tin atoms (Sn). The conversion of the tin oxides may be described as follows:

SnO_(x)+yF→SnO_(x)F_(y)   (Eq. 3)

[0019] In addition to the existence of oxyfluorides in the oxyfluoride layer 19 of treated solder balls 16, there may also be some tin-fluoride compounds (for example, SnF₂ and Sn₂F₆).

[0020] As seen in the graph of FIG. 4, the plasma product treatment significantly reduces the atomic concentration of oxygen on the surface of a treated solder ball 16 as well as the penetration of the oxygen into the treated solder ball 16 in comparison to solder balls that are not treated. For example, the atomic concentration of oxygen on the surface of a treated solder ball 16 is about 34% while the atomic concentration of oxygen on the surface of a newly produced untreated solder ball 15 is about 41%. In addition, the penetration of oxygen in a treated solder ball 16 is about 60 Å deep while the penetration of oxygen in a newly produced untreated solder ball 15 is about 90-100 Å deep. The graph of FIG. 5 depicts the relative atomic concentrations of lead (Pb), tin (Sn), fluorine (F), oxygen (O), and carbon (C) relative to solder ball depth for plasma product treated 63 Sn/37 Pb solder balls 16.

[0021] Once treated, the oxyfluoride layer 19 (FIG. 2) on the treated solder balls 16 allows the treated solder balls 16 to be reflowed or soldered in a ball grid array without the use of flux for cleaning the solder balls 16. For purposes of description, soldering includes reflowing. The thin oxyfluoride layer 19 formed on the treated solder balls 16 has a structure that is sufficiently brittle to fracture or break up into small pieces when the treated solder balls 16 melt, allowing clean solder to flow, thereby permitting reflow as well as proper joining. Consequently, the use of the treated solder balls 16 can eliminate the steps of applying flux prior to soldering and then cleaning flux residues afterwards. The oxyfluoride layer 19 also limits subsequent oxide growth 17 on the treated solder balls 16 so that the treated solder balls 16 can be stored for a period of time in air (for example, about six days) before use. After about seven days, sufficient oxide growth may form on the treated solder balls 16 to prevent fluxless soldering. When performing a reflow process with treated solder balls 16, a rapid reflow which occurs in about five seconds or less provides the best results because there is little time for sufficient oxides to grow to hamper the reflow process. If desired, the treated solder balls 16 can also be used with various fluxes such as water soluble flux, no-clean flux, and rosin-based flux.

[0022] A more detailed description of solder ball apparatus 10 now follows. Referring to FIG. 6, solder droplet generator 12 includes a housing 35 in which the lower portion forms a crucible 40 where solder 38 is melted and contained. Heater 42 extends around crucible 40 for heating and melting the solder 38 contained within crucible 40. A thermocouple 33 monitors the temperature of the molten solder 38 for maintaining the proper temperature. For a 63 Sn/37 Pb solder composition, the molten solder 38 may be maintained at about 235° C. A piezoelectric actuator 32 is clamped against the upper disk 36 a of a vibration transmitting member 36 at the upper portion of housing 35 by a clamping plate 37 and bolts 39. The upper disk 36 a is positioned over an opening 33 at the upper portion of housing 35 with the piezoelectric actuator 32 clamped against the top surface of the upper disk 36 a. Vibration transmitting member 36 transmits vibrations produced by piezoelectric actuator 32 to the molten solder 38. Vibration transmitting member 36 includes a shaft 36 b extending downwardly from upper disk 36 a which is connected to a lower disk 36 c. The lower disk 36 c is extended into the lower portion of crucible 40 within the molten solder 38. Vibrations produced by piezoelectric actuator 32 are transferred downwardly through upper disk 36 a and shaft 36 b to the lower disk 36 c of vibration transmitting member 36 for perturbing the molten solder 38.

[0023] Pressurized gas, for example, an inert gas such as nitrogen, argon or helium, is employed to pressurize the interior of housing 35 via inlet 34. The pressurized gas is employed for forcing molten solder 38 from crucible 40 through the orifice 44 located at the bottom of crucible 40. A pressure differential of only about 35 kPa (5 lb./in.²) is required to force a falling jet of molten solder 45 from crucible 40 through orifice 44, however, a pressure differential of between about 135-700 kPa (20-100 lb./in.²) is preferred. By vibrating piezoelectric actuator 32 at a periodic oscillation having a wave length greater than the circumference of the jet diameter, the falling jet 45 of molten solder 38 breaks into a train of solder droplets 47 while falling. The droplets 47 pass through an opening 46 a in a charging plate 46 located below the crucible 40. The charging plate is provided with a voltage which charges the falling droplets 47 by electrostatic induction to prevent merging of the droplets 47 during flight.

[0024] The diameter of orifice 44, the pressure differential within crucible 40 and the vibration frequency of piezoelectric actuator 32, varies depending upon the size of the solder balls 16 to be made. For example, an orifice 44 diameter of 406 μm, a pressure differential of 34.4 kPa and a vibration frequency of 1430 Hz may be used to produce solder balls 15 that are 760 μm in diameter; an orifice 44 diameter of 254 μm, a pressure differential of 44.8 kPa, and a vibration frequency of 2582 Hz may be used to produce solder balls 15 that are 500 μm in diameter; and an orifice 44 diameter of 178 μm, a pressure differential of 68.9 kPa, and a vibration frequency of 4370 Hz may be used to produce solder balls 15 that are 300 μm in diameter. In order to obtain a particular solder ball 15 diameter, shrinkage of the solder while cooling is also taken into account. The different orifice 44 diameters and pressure differentials within crucible 40 provides different initial velocities of the jet 45 of molten solder. For example, an orifice 44 diameter of 406 μm and a pressure differential of 34.4 kPa provides an initial jet velocity of 2.8 m/s; an orifice 44 diameter of 254 μm and a pressure differential of 44.8 kPa provides an initial jet velocity of 3.2 m/s; and an orifice 44 diameter of 178 μm and a pressure differential of 68.9 kPa provides an initial jet velocity of 4.1 m/s. For producing solder balls that are small in diameter, the higher initial jet velocities in combination with the small ball diameters allows over a million solder balls to be produced in just a five-minute period of time.

[0025] During the operation of solder droplet generator 12, variations in the target diameter of the solder balls 15 may be controlled by a closed loop control system where the size of the falling solder droplets 47 is measured by a CCD camera and the vibration frequency of the piezoelectric actuator 32 adjusted in response to the measurements. Typically, the solder droplets 47 are measured by digital image analysis where the images from the CCD camera are transformed into a pixel array and then the pixel values are transformed into length units. The CCD camera is calibrated to account for any optical distortion and image amplification. Such a control system can produce solder spheres with a size variation smaller than ±2.5% of the target size.

[0026] Although solder droplet generator 12 has been shown and described for use in solder ball apparatus 10, other suitable molten droplet generators may be employed, for example, the devices described in U.S. Pat. Nos. 5,266,098 and 5,431,315, the contents of which are incorporated herein by reference in their entirety.

[0027] Referring to FIG. 7, plasma generator 14 is a microwave plasma generator which dissociates the SF₆ gas with microwaves. Plasma generator 14 may be formed from a microwave oven 14 a within which a Pyrex® inner vessel 30 is mounted. The power of plasma generator 14 is controlled by controls 54. An iron oxide polymer and aluminum mesh is used to prevent microwaves from radiating to the outside environment from plasma generator 14. Two flow meters 48 a and 48 b with respective inlets 26 a and 26 b are coupled in communication with inlet 26. The flow of N₂ gas and SF₆ gas into inner vessel 30 is controlled by respective flow meters 48 a and 48 b. A pirani type pressure gauge 52 measures the pressure within inner vessel 30. Inlet 26 and conduit 28 include elongate tubes 31 extending within the interior 30 a of inner vessel 30 for delivering the gases to and removing the plasma products 13 a containing fluorine radicals F⁺ from the center of the microwave oven 14 a. Pressure gauge 52 also includes an elongated tube 31 for measuring the pressure at this central region. The length of conduit 28 is kept to a minimum and is coupled to chamber 18 at a position for quickly delivering plasma products 13 a to the falling solder balls 15 in order to minimize the recombination of the fluorine radicals F⁺ with neutral species before contacting the solder balls 15.

[0028] A stable plasma 13 may be generated by plasma generator 14 at a power of 1000 watts, a frequency of 2.45 GHz and an SF₆ gas pressure of 0.15 to 5 Torr. Typically, microwave power above 600 watts provides maximum dissociation of SF₆ gas. The higher SF₆ gas pressures are preferred to provide a higher concentration of atomic fluorine F⁺. A high concentration of atomic fluorine is desirable to ensure sufficient treatment of solder balls 15 because the treatment time of the falling solder balls 15 is very short. The pressure of the plasma 13 within the system may be controlled by the SF₆ gas flow rate. For example, a plasma 13 pressure of 0.8 Torr may be obtained by a SF₆ gas flow rate of 100 SCCM (standard cubic centimeters per minute), a plasma 13 pressure of 1 Torr may be obtained by a SF₆ gas flow rate of 476 SCCM, a plasma 13 pressure of 1.5 Torr may be obtained by a SF₆ gas flow rate of 985 SCCM, and a plasma 13 pressure of 3 Torr may be obtained by a SF₆ gas flow rate of 1302 SCCM.

[0029] While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.

[0030] For example, although solder ball apparatus 10 is depicted to position solder ball generator 12 in a chamber 18 that receives plasma products 13 a containing fluorine radicals F⁺ from plasma generator 14 via conduit 28, alternatively, plasma generator 14 may be modified to house solder ball generator 12 within vessel 30 so that both the plasma 13 and the solder balls are produced in vessel 30. In such a case, solder ball generator 12 must be shielded from the microwaves. In addition, although plasma generator 14 is described as a microwave plasma generator, alternatively, plasma generator 14 may generate plasma by other suitable methods, such as by radio frequency. Furthermore, although conduit 28 preferably is mounted to the top of chamber 18 for delivering plasma products 13 a into chamber 18, alternatively, conduit 28 may be mounted at other suitable locations, with the vacuum fitting 24 also being positioned in an appropriate position relative to conduit 28 to maintain an adequate plasma concentration in the chamber 18. Although the present invention has been described for treating solder balls having a composition of 63 Sn/37Pb, it is understood that solder balls of other compositions may be treated, such as 10 Sn/90 Pb solder balls. Also, chamber 18 may be employed for treating solder balls that have been previously formed. The solder balls may be loaded into a feed device which drops the solder balls through the plasma product 13 a filled chamber 18. The solder balls may be stored in an inert environment before and/or after treatment. The container 20 for collecting treated solder balls may be replaced by a conveyance system. Finally, solder forms not necessarily spherical in shape may also be formed and treated with plasma products 13 a. 

What is claimed is:
 1. A method of forming fluxless solder balls comprising: forming solder balls from a supply of solder; and forming a coating on the solder balls for limiting naturally occurring oxide growth on the solder balls before significant natural oxide growth on the solder balls has occurred, the coating allowing the solder balls to be soldered without using flux.
 2. The method of claim 1 further comprising forming the solder balls from molten solder.
 3. The method of claim 2 further comprising forming the solder balls by a droplet spray process wherein the molten solder is caused to fall as droplets which solidify while falling to form the solder balls.
 4. The method of claim 3 further comprising forming the coating on the solder balls while the solder balls are falling.
 5. The method of claim 4 in which forming the coating on the solder balls comprises treating the solder balls with plasma products.
 6. The method of claim 5 further comprising including fluorine within the plasma products.
 7. The method of claim 6 further comprising forming an oxyfluoride layer on the solder balls.
 8. The method of claim 5 further comprising forming the solder balls at a solder ball forming station.
 9. The method of claim 8 further comprising coating the solder balls at a coating station.
 10. The method of claim 9 in which the coating station comprises a chamber containing the plasma products therein.
 11. The method of claim 10 further comprising supplying the chamber with the plasma products from a plasma generator.
 12. The method of claim 11 further comprising supplying the plasma generator with a gas containing fluorine.
 13. A method of forming fluxless solder balls comprising: forming solder balls by a droplet spray process wherein molten solder is caused to fall as droplets which solidify while falling to form the solder balls; and forming a coating on the solder balls before significant natural oxide growth on the solder balls has occurred by treating the solder balls with plasma products while the solder balls are falling, the coating limiting naturally occurring oxide growth on the solder balls and allowing the solder balls to be soldered without using flux.
 14. The method of claim 13 further comprising including fluorine within the plasma products.
 15. The method of claim 14 further comprising forming an oxyfluoride layer on the solder balls.
 16. The method of claim 13 further comprising forming the solder balls at a solder ball forming station.
 17. The method of claim 16 further comprising coating the solder balls at a coating station.
 18. The method of claim 17 in which the coating station comprises a chamber containing the plasma products therein.
 19. The method of claim 18 further comprising supplying the chamber with the plasma products from a plasma generator.
 20. The method of claim 19 further comprising supplying the plasma generator with a gas containing fluorine.
 21. A method of forming fluxless solder forms comprising: forming solder forms from a supply of solder; and forming a layer on the solder forms for limiting naturally occurring oxide growth on the solder forms, the layer allowing the solder forms to be soldered without using flux.
 22. An apparatus for forming fluxless solder balls comprising: a solder ball forming station for forming solder balls from a supply of solder; and a coating station for forming a coating on the solder balls for limiting naturally occurring oxide growth on the solder balls before significant natural oxide growth on the solder balls has occurred, the coating allowing the solder balls to be soldered without using flux.
 23. The apparatus of claim 22 in which the solder ball forming station forms the solder balls from molten solder.
 24. The apparatus of claims 23 in which the solder ball forming station forms falling droplets of molten solder which solidify while falling to form the solder balls.
 25. The apparatus of claim 24 in which the coating station is positioned below the ball forming station for forming the coating on the solder balls while the solder balls are falling.
 26. The apparatus of claim 25 in which the coating station treats the solder balls with plasma products.
 27. The apparatus of claim 26 in which the plasma products include fluorine for forming an oxyfluoride layer on the solder balls.
 28. The apparatus of claim 26 in which the coating station comprises a chamber containing the plasma products therein.
 29. The apparatus of claim 28 further comprising a plasma generator for producing plasma and supplying the chamber with the plasma products.
 30. The apparatus of claim 29 further comprising a gas source for supplying the plasma generator with a gas containing fluorine.
 31. An apparatus for forming fluxless solder balls comprising: a solder ball forming station for forming falling droplets of molten solder which solidify while falling to form solder balls; and a coating station positioned below the solder ball forming station for forming a coating on the solder balls by treating the solder balls with plasma products while the solder balls are falling and before significant natural oxide growth on the solder balls has occurred, the coating limiting naturally occurring oxide growth on the solder balls and allowing the solder balls to be soldered without using flux.
 32. The apparatus of claim 31 in which the coating station treats the solder balls with plasma products.
 33. The apparatus of claim 32 in which the plasma products include fluorine for forming an oxyfluoride layer on the solder balls.
 34. The apparatus of claim 32 in which the coating station comprises a chamber containing the plasma products therein.
 35. The apparatus of claim 34 further comprising a plasma generator for producing plasma and supplying the chamber with the plasma products.
 36. The apparatus of claim 35 further comprising a gas source for supplying the plasma generator with a gas containing fluorine.
 37. A fluxless solder ball for a ball grid array comprising a generally round ball of solder, the ball of solder having a thin brittle oxyfluoride layer which limits naturally occurring oxide growth on the surface of the solder ball, during soldering, the oxyfluoride layer is capable of fracturing to allow the solder to flow, thereby permitting soldering without using flux. 